1. Field of the Invention
The present invention relates to wireless communications systems, and, in particular to antenna arrays used by such systems that employ multiple polarizations.
2. Description of the Related Art
Multiple-input multiple-output (MIMO) technologies are well known in the field of wireless communications. In general, these technologies use multiple antennas at both the transmitter and the receiver of a communications system to perform communications. Communications systems that employ MIMO technologies may achieve certain performance improvements over single-input single-output (SISO) systems that use only one antenna at both the transmitter and the receiver.
For example, in fading environments, MIMO technologies may be used to improve reliability of communications. In such environments, a transmitted signal may travel over multiple different propagation paths to a receiver antenna. Each propagation path carries a version of the transmitted signal that is altered due to factors such as the length of the path, the number of reflections in the path, and the characteristics of any objects in the path. These factors may vary from one path to the next, and, as a result, each version of the transmitted signal may arrive at the receiver antenna with a delay, a signal attenuation, and a phase shift that are different from those of the other versions. As the multiple versions arrive at the receiver antenna, they may constructively or destructively interfere with one another such that the signal received by the receiver is an amplified or attenuated version of the transmitted signal. If attenuation is relatively severe, then errors may result when decoding the received signal, thereby preventing the receiver from recovering the communicated information.
MIMO technologies may be used to prevent these errors from occurring. In particular, MIMO technologies may be used to create multiple “diverse” copies of the information being communicated so that the receiver may have multiple opportunities to decode the information. When one or more copies experience relatively severe attenuation, the communicated information may be recovered from the remaining copies. Further, the receiver may combine all of the copies in an optimal way to recover as much of the communicated information as possible.
As another example, MIMO technologies, such as the Bell Labs Layered Space-Time (BLAST) technology, may be used to increase capacity of communications performed over a limited bandwidth. To achieve improved capacity, the information to be transmitted is divided into a number of separate information streams, where the number of separate information streams is equal to the number of transmitter antennas. The separate information streams are transmitted via different antennas to the receiver where they are decoded and reassembled into the original information generated at the transmitter. In essence, using MIMO technologies in such a manner creates parallel communications channels without requiring additional bandwidth. It is possible to construct a communications system with a transmission capacity that increases linearly as the number of transmitting and receiving antennas increase. For example, a MIMO system having two antennas at both the transmitter and the receiver may be capable of achieving double the capacity of a SISO system having only one antenna at both the transmitter and the receiver.
To achieve the advantages described above, a MIMO system should be capable of distinguishing between the signals transmitted via the multiple transmitter antennas. This may be accomplished by spacing the transmitter and receiver antennas apart such that the set of propagation paths generated by each transmitter antenna is different from those generated by the other transmitter antennas. The distance between the antennas may range from one-half the transmitter's operating wavelength to several operating wavelengths. These spacing requirements may make it difficult, if not impossible, to implement multiple antennas within a relatively small communications device.
The advantages of MIMO may be achieved for relatively small communications devices by employing polarization diversity. In polarization diversity, each of the multiple transmitter antennas transmits a signal using an antenna polarization that is different from that of the other antennas. Multiple antennas having polarizations that are orthogonal to one another may be placed together (i.e., co-located) and are not restricted by spacing requirements. As discussed in Andrews, “Tripling the capacity of wireless communications using electromagnetic polarization,” Letters to Nature, Vol. 409, 18 Jan. 2001, pages 316-318, the teachings of which are incorporated herein by reference in their entirety, a MIMO system may be implemented using as many as three differently-polarized, co-located antennas.
Multi-polarized antenna arrays may also be used in wireless communications systems that employ polarization diversity without using MIMO technologies. For example, in wireless communications systems that transmit only one copy of a signal, the polarization of the signal relaying that single copy might not be aligned with the polarization of the receiver antenna. This misalignment may cause signal reception to be less reliable. To improve reliability in such cases, a multi-polarized antenna array may be used at the receiver. The multi-polarized antenna array receives multiple differently-polarized versions of the transmitted signal. The multiple differently-polarized versions may then be combined in a manner similar to the combining techniques employed for spatial diversity to improve reliability. Alternatively, the receiver may select the more reliable of the differently-polarized versions for decoding.